1. Field of the Invention
The invention relates to a device for acquisition of logic states from multiple sensors operating in all-or-nothing mode.
2. Discussion of the Background
The principal, but not exclusive, application of the invention is the detection of malfunctions in all parts of an aircraft and the indication of these malfunctions at a place which is visible to the pilot of the aircraft.
Conventionally, the practice was to link a sensor operating in all-or-nothing mode, that is to say an electrical switch, in practice, to a pilot light the color of which, for example red, orange or green, represented the level of seriousness of the malfunction.
With the growth in the complexity of operation of aircraft, it is becoming necessary to arrange several tens of alarm lamps, and it is desired to replace all these lamps by a single visual-display console displaying, on a screen, an indication as to the nature of the malfunction only when a malfunction is observed. The color of the display again corresponds to the level of seriousness of the malfunction.
It is desirable for this console to be usable in all types of aircraft, and it should therefore be able to be adapted to different numbers and different types of malfunction sensors.
With the sensors being switches (door-opening detector, detector of position of movable members; etc), a conventional solution then consists in establishing a logic signal representing the open or closed state of each switch and in transmitting this logic state to one of the multiple inputs of a multiplexer. This multiplexer is addressed sequentially and sends, to its output, at a given instant, the open or closed logic state of the switch which is addressed at that instant. The information at the output of the multiplexer is managed by a controller in order to display indications in the event of a malfunction being detected by a sensor.
If a switch is open, the logic state at the output of the multiplexer will, for example, be the zero state; if it is closed, it will be the xe2x80x9c1xe2x80x9d state (the inverse is also possible). The switch is generally open at rest, closed in the event of a malfunction being detected (for reasons of reduction in the consumption at rest). The multiplexer consists of addressable logic gates arranged between the inputs and the output.
This requires the switch to be linked, upstream of the logic multiplexer, between two voltage levels, via a pulling resistor, in such a way that the potential of the junction point between the switch and the resistor changes from one potential level to the other depending on whether the switch is open or closed. One of the potential levels corresponds to a logic high state, the other to a logic low state. The pulling resistor may be a pull-up resistor or pull-down resistor.
The junction point of the switch and of its pulling resistor is linked to a threshold-type comparator which switches over in one direction or the other depending on whether the switch is open or closed, and the output of the comparator is linked to one input of the multiplexer.
One of the difficulties in implementation stems from the lack of reliability of detection of the open or closed state of the switch upstream of the multiplexer. This is because this detection depends:
on the value of the pulling resistor,
on the intrinsic resistance of the contact of the switch,
on the power-supply potential of the combination of the switch plus pulling resistor,
on the variations in these values with temperature, etc.
Typically, in an aircraft or a helicopter, it is possible to use the general electrical earth of the aircraft as low-potential level and the general DC electrical power supply of the aircraft, for example at 28 volts, as high-potential level.
However, this power supply is not stable. It varies over time, it is subject to overvoltages, to microbreaks, to standby batteries being or not being put into service, etc.
The schedules of specifications dictate, for example, that, for a nominal value of 28 volts, the apparatus operates correctly even if the voltage falls to 16 volts or rises to 36 volts.
The pulling resistor depends on the type of sensor: some require a fairly low resistance, for example because the sensor is in series with a local pilot light; others use a higher resistance in order to limit the power consumption.
The contact resistance of the switch varies with its aging: dirtying of the contacts, wear. However, the potential applied to the input of the threshold-type comparator, in the closed state of the switch, depends on the ratio between the resistance value of the switch proper and the value of the pulling resistor.
For these reasons, and for yet more, it is not easy to set the threshold or thresholds of the comparators individually so as to be certain that the logic state transmitted by the comparator will correspond to an open or closed state of the switch.
Moreover, it would be desirable to have not just information on the open or closed state of the switch, but also information on the fact that this information is not reliable and should not be taken into account (for example because of the aging of a sensor: failure information, implying the need for replacement, would be desirable). The same would be true, moreover, of information on a circuit fault (fault in a comparator, multiplexer, connection, etc.) between the sensor and the visual-display screen controller.
Furthermore, it is desirable to have the most flexible and the most universal acquisition system possible, allowing for numerous configurations for detection of malfunctions by simple programming of parameters internal to the system, with a minimum number of modifications to electronic cards in order to change from one application to another.
Finally, clearly, the constraints of cost, of bulk and of weight are an important element to be taken into consideration, as well as the possibilities for frequent testing of the system, above all for aeronautical applications.
Hence the invention proposes a system for acquisition of logic states from multiple sensors operating in all-or-nothing mode, in which each sensor consists of a switch linked in series with a pulling resistor between earth and a power-supply potential, this system being characterized in that the junction point of a sensor and of the corresponding resistor is linked to a corresponding input of an analog multiplexer, a voltage representative of the power-supply potential also being linked to a specific input of the multiplexer, the output of the multiplexer being linked to an analog-digital converter, the multiplexer and the converter being driven by a calculating logic unit which periodically and sequentially generates all the input addresses of the multiplexer, reads and stores the corresponding digital values at the output of the converter for each address, determines a digital value of power-supply voltage, determines, for each address corresponding to a sensor, the position of the digital value read for this address with respect to at least one respective threshold value relating to this address, this position representing the logic state of the sensor, means being provided for altering the threshold value as a function of the digital value determined for the power-supply voltage.
It may be wished to detect not only the open or closed state of the sensor but also a range of uncertainty for which the state has to be regarded as indeterminate (sign of failure of the sensor or of a part of the link for detecting the state of the sensor). In this case, the position of the digital value corresponding to the address of a sensor is compared with two thresholds, the indeterminate state corresponding to a value lying between the two thresholds. Each of these thresholds can be altered as a function of the digital value attributed to the power-supply voltage by the system.
If the acquisition system is used to display malfunctions on a visual-display screen, no indication will be displayed if the switch is in rest position (open state, in general), specific alarm information for the sensor concerned will be displayed in the event of the inverse state (switch closed), and fault information in the case of an uncertain state.
The threshold values for a given sensor and for a normal power-supply voltage value are preferably stored in a programmable memory of the logic unit.
The threshold values used in the comparison are calculated for each sensor as a function of the current digital value determined by the system for the power-supply voltage to the sensors (in general one voltage for all the sensors, but, if there are several power-supply voltages, each voltage will be applied to a respective input of the analog multiplexer in order to be measured).
The variation in the threshold voltages as a function of the power-supply voltage may follow any law, programmed within the logic unit, but in general a proportional variation will be sufficient.
The digital value of power-supply voltage used to alter the thresholds will preferably be a filtered value rather than the instantaneous value read in the course of a sequence of reading and analysis of the analog voltages output by the sensors. This makes it possible to avoid disturbances (and thus false alarms, for example) due to microbreaks in power supply or to insignificant, very brief, stray overvoltages.
In practice, the calculating logic unit comprises a sequencer driving, in synchronism, the addressing of the multiplexer, the analog-digital converter, and a memory for storing the values supplied by the converter, and a programmed microcontroller capable of addressing the storage memory in order to extract from it the information stored by the sequencer, to process this information, and to supply information resulting from this processing and representing the state of each of the sensors.